All-fused Silica 248-nm Lithographic Projection Lens

J. Brian Caldwell

The objectives used in deep UV
photolithography to image integrated circuit patterns onto silicon wafers are among the most sophisticated imaging systems ever developed. These systems are typically diffraction limited at an NA of 0.5 or more over a fairly large image area, with essentially zero distortion. In
addition, there is normally a requirement that they be telecentric in both object and image spaces so that any small amount of defocus caused by
curvature of the object or image
planes will not result in the introduction of distortion. It is important to keep distortion to an absolute minimum because these objectives are used as part of a stepper system in which a wafer is exposed in numerous discrete steps. Any distortion in the objective will result in a slight
mismatch of the tiny circuit features in adjacent exposures. Thus, it is necessary to keep the image displacement due to distortion below a small fraction of the smallest feature size.